[simple theme] create favicon.png from src/svg/searxng-wordmark.svg

new bash function convert_if_newer() / usage::

    convert_if_newer <origfile> <outfile> [<options>, ...]
    convert_if_newer "path/to/origin.svg" "path/to/converted.png" -transparent white -resize 64x64

Run's ImageMagik' convert comand to generate <outfile> from <origfile>, if
<origfile> is newer than <outfile>.  The command line is to convert is::

    convert <origfile> [<options>, ...] <outfile>

PNG 'searx/static/themes/simple/img/favicon.png' has been created by::

  $ make themes.simple
  CONVERT   searx/static/themes/simple/src/svg/searxng-wordmark.svg -transparent white -resize 64x64 searx/static/themes/simple/img/favicon.png
  ...

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>